This invention relates to methods for recovering tungsten from tungsten containing spent catalysts which involve digesting the catalyst in sodium hydroxide using a relatively low ratio of sodium hydroxide to tungsten. At least about 77% by weight of the tungsten can be recovered as sodium tungstate under these conditions. The remainder of the tungsten can be recovered by first grinding the catalyst and increasing the sodium hydroxide to tungsten mole ratio.
Desulfurization catalysts using valuable metals such as tungsten, molybdenum, cobalt, and nickel, as components are used in the petroleum refining industry. Generally the catalysts are made of a carrier such as aluminum oxide, spinel (MgO.Al.sub.2 O.sub.3), or aluminum silicate.
Due to economic and environmental factors, it is feasible to recover the valuable metals from these spent catalysts for reuse.
One method of recovery involves the use of a soda ash digestion followed by considerable processing to a calcium tungstate end product. This method requires the use of acid to drive off the carbonate and calcium to precipitate the tungsten. The final form of the tungsten product is also of limited usefulness.
U.S. Pat. No. 4,384,885 relates to another method in which an oxidative roasting followed by carbon/CaO reduction is utilized. The tungsten is found in the metal slag of this process.
Bureau of Mines (Document No. RI 9252, B. W. Jong et al) teaches that NaOH can be used in mole ratios of NaOH to W exceeding 3.5. In particular they claim that mole ratios of 7-14 are necessary in order to attain tungsten dissolution efficiencies of 85-90%.
The present invention provides methods for efficiently recovering tungsten from such spent catalysts.